1. Field of the Invention
The present invention concerns a device for changing colour during the extrusion of a sheath of synthetic material around a conductor, comprising an extruder head provided with at least two inlet conduits for synthetic material, connected respectively to two orifices traversed by a conductor to allow extruding around the latter a sheath formed of two layers, and distributing means for selectively connecting these inlet conduits to a first and a second extruder of snythetic material.
2. Description of the Related Art
In addition to the great variety in the types of insulated electric wires and cables called for by the utilizers from the cable making industry, each type of cable or wire may have a sheath of various colours for purposes of identification of the conductors. Up to eleven colours may be utilized, alone or in combination. Now, the utilizers often require the manufacturers, notably in the field of special cables, to produce relatively small quantities of wires insulated by a sheath of a determined colour, which poses enormous production problems. As a matter of fact, it is practically impossible to stock all types of products and in all of the colours.
In a line for insulating by extrusion of a sheath of synthetic material, each time the colour of the sheath has to be changed, the extruder heads and the extruders which feed them with the synthetic material under pressure must be entirely purged, in order that the preceding colour may not influence the new colour of the sheath extruded onto the conductor. This purging operation generally neccessitates stopping the insulating line, which means a significant loss in the output of the production equipment. In addition, this purging brings about an important amount of wastes, which are generally not reutilizable since their colour is not pure. Now, these raw materials are relatively costly and represent an important part of the cost of the final product. Consequently, each colour changing operation increases the unit price of the product, particularly for a product made in small quantity.
A solution allowing to avoid stopping production consists in charging into the extruder the synthetic material of the new colour while continuing to produce a sheath of the old colour with the material remaining in the machine. However, a relatively significant amount of material having a mixed colour nevertheless subsists and has to be discharged either by purging while stopping production, or by producing a reject sheath.
Another solution is provided by the patent application DE-B No. 1,222,656, which describes a device of the type indicated in the preamble. The two extruders are disposed on both sides of the head and are each selectively connected to two respective inlet conduits each corresponding to one layer of the sheath, via a distributor with a pivoting plug which directs the material towards one of the conduits, the other then not being utilized. There are thus two distributors and four inlet conduits. To effect changing of the apparent colour of the sheath, the material having the new colour is previously charged into the extruder which feeds the inner layer and the extrusion is pursued until this material has completely replaced the preceding one in the extruder and in the extruder head. The production line is then briefly stopped and the two distributors are actuated to interchange the feeds, the material having the new colour thus passing from the inner layer to the outer apparent layer of the sheath when the extrusion recommences.
A drawback of this device is the fact that the production line has to be stopped. Another drawback consists of the inlet conduits which are not utilized, during a period which may be quite long. As the material which stagnates in these conduits can not be purged, it will produce a certain length of reject sheath at the next colour change, but it particularly tends to be decomposed, which may foul up or clog the conduits or the dies of the extruder head. Finally, the described construction is relatively complicated and neccessitates means for ensuring the synchronism of the two distributors.